The present invention relates to a method for operating a fuel injector.
An electromagnetically operable fuel injector for the direct injection of fuel into the combustion chamber of an internal combustion engine is described in German Patent Application No. 196 26 576. In this fuel injector, an armature cooperates with an electrically energizable magnetic coil for the electromagnetic actuation. The lift of the armature is transferred via a valve needle to a valve-closure member. The valve-closure member cooperates with a valve-seat surface to form a sealing seat. The valve needle and the valve-closure member are reset by a resetting spring.
The relatively long closing times are disadvantageous in the fuel injector described in German Patent Application No. 196 26 576 A1. Delays in closing the fuel injector are caused by the adhesive powers acting between the armature and the core, and the non-instantaneous decay of the magnetic field in response to switching off the excitation current. Therefore, the resetting spring must have a large spring constant or a great bias (prestressing). To-achieve shorter closing times, the restoring force must be dimensioned to be substantially greater than would be necessary for sealing against the combustion chamber pressure. This involves a great power demand of the electronic trigger circuit.
A method according to the present invention for operating a fuel injector has the advantage that an additional current pulse at the end of the opening phase has a positive effect on the closing operation. In the final phase of the opening interval, the total spring energy acting in the closing direction is increased by the additional current pulse.
Due to the additional cutoff spring, an additional accelerative force is available during the closing operation to quickly close the fuel injector. The spring constant of the resetting spring is dimensioned in such a way that the spring energy exerted still safely suffices to seal the fuel injector against the pressure in the combustion chamber of the internal combustion engine.
The method is particularly advantageous in the low speed range of the internal combustion engine, since in this range, one strives for the metering of small amounts of fuel at relatively long time intervals.
Thermal overloading of the fuel injector and of the electrical component is virtually ruled out, since the current pulses are only supplied over very short periods of time with long pauses between.
By supplying a current pulse, the magnetic field is built up again to a higher magnetic-field value, which offers the advantagexe2x80x94relatively consideredxe2x80x94of the rapid reduction in the periods of time relevant for the closing operation, since the magnetic field decreases approximately exponentially with the time.
The cutoff spring can be replaced by applying an elastically deformable layer on the armature stop face of the core and/or of the armature, since deformation energy can be stored in the elastically deformable layer which acts like a spring with a very high spring constant. This energy is available again for the closing operation.